text: Molecules with polyhapto ligands are often fluxional, also known as stereochemically non-rigid. Two classes of fluxionality are prevalent for organometallic complexes of polyhapto ligands:
  *Case 1, typically: when the hapticity value is less than the number of sp carbon atoms. In such situations, the metal will often migrate from carbon to carbon, maintaining the same net hapticity. The η-CH ligand in (η-CH)Fe( η-CH)(CO) rearranges rapidly in solution such that Fe binds alternatingly to each carbon atom in the η-CH ligand. This reaction is degenerate and, in the jargon of organic chemistry, it is an example of a sigmatropic rearrangement.  A related example is Bis(cyclooctatetraene)iron, in which the η- and η-CH rings interconvert.
  *Case 2, typically: complexes containing cyclic polyhapto ligands with maximized hapticity. Such ligands tend to rotate. A famous example is ferrocene, Fe(η-CH), wherein the Cp rings rotate with a low energy barrier about the principal axis of the molecule that "skewers" each ring (see rotational symmetry). This "ring torsion" explains, inter alia, why only one isomer can be isolated for Fe(η-CHBr) since the torsional barrier is very low.